1. Technical Field
The present invention relates in general to an improved fluorescent lamp and in particular to an extended temperature range fluorescent lamp. Still more particularly, the present invention relates to an extended temperature range fluorescent lamp which includes a volume variation control for varying the volume of ionizable medium within the fluorescent lamp in response to temperature variations.
2. Description of the Related Art
Fluorescent lamps, as well known in the prior art, typically comprise a sealed glass envelope containing an ionizable medium, such as neon or argon and a small amount of mercury. An electronic discharge between electrodes at each end of the sealed glass envelope vaporizes and ionizes the mercury and excites the mercury to ultraviolet radiation which, in turn, causes a phosphor to radiate visible light by fluorescence.
It is also well known that the light output of fluorescent lamps is directly dependent upon ambient temperature. This dependence arises from the fact that vapor pressures within a fluorescent lamp depend upon the temperature of the coolest part of the lamp, which in turn depends upon the temperature of the air in which the lamp is operating.
The current state-of-the-art with respect to fluorescent lamps utilizes a constant volume of gas within a tube and the pressure therein will then vary as a function of the ambient temperature. Resultant decreases in light efficiency are caused by pressures which are below the optimum point at low temperatures and pressures which are above the optimum point at high temperatures.
Numerous attempts have been made to compensate fluorescent lamps for changes in ambient temperature. For example, U.S. Pat. No. 3,284,664 teaches pressure regulation of the ionizable medium within a fluorescent lamp by the utilization of Peltier cooling devices, such as thermoelectric junctions. By providing a thermoelectric junction, on the surface of the fluorescent tube, heat may be. added to or subtracted from the fluorescent lamp in order to compensate the lamp for variations in ambient temperature.
U.S. Pat. No. 3,617,792 teaches a fluorescent lamp in which the discharge is confined to an inner vitreous tube within a sealed outer envelope in an effort to stabilize the fluorescence within the lamp. U.S. Pat. No. 3,246,189 teaches the utilization of an auxiliary electrode which is connected to one emissive electrode and located between a wall between the electrodes which provides a chamber for one electrode and forms a passage for electron discharge from one electrode to the other electrode, such that ion flow will be inhibited, thereby raising the vapor pressure of the ionizable material within the chamber to compensate for low ambient temperatures.
Additionally, increases in electrical power applied to such lamps have been proposed for utilization in high temperature environments. However, since the increase in electrical power involves greater power dissipation and further increases the temperature within the fluorescent lamp, a further decrease in efficiency and a possible thermal runaway condition may result.
Fluorescent lamp technology is currently proposed for utilization within flat panel display back light applications because of the high efficiency, uniform distribution and xe2x80x9cflatxe2x80x9d aspect ratio of such lighting. However, rigorous military and outdoor, rugged environment applications for such flat panel technology require the provision of a fluorescent lamp which provides uniform lighting over an extended temperature range.
It should therefore be apparent that a need exists for an extended temperature range fluorescent lamp.
It is therefore one object of the present invention to provide an improved fluorescent lamp.
It is another object of the present invention to provide an extended temperature range fluorescent lamp.
It is yet another object of the present invention to provide an extended temperature range fluorescent lamp which includes a volume variation control for varying the volume of ionizable material within the fluorescent lamp in response to ambient temperature variations.
The foregoing objects are achieved as is now described. An extended temperature range fluorescent lamp is provided which includes an envelope which contains an ionizable medium at a selected pressure. An electrode at each end of the envelope is then utilized to sustain an electric discharge through the ionizable medium. Variations in temperature of the fluorescent lamp cause variations in vapor pressure within the envelope and adversely effect the lighting efficiency of the fluorescent lamp. A volume variation control is provided which varies the volume of the ionizable medium within the envelope in response to temperature variations, such that a selected pressure can be maintained. Volume variation is accomplished by providing a flexible mechanical interface-to the envelope or by coupling one or more control volumes of ionizable medium to the envelope and selectively varying the volume of ionizable medium within the envelope in response to temperature variations.
The above as well as additional objectives, features, and advantages of the present invention will become apparent in the following detailed written description.